1. Field of the Invention
The invention relates to a light-emitting diode (LED) driving circuit. More particularly, the invention relates to an LED current balance circuit for driving a plurality of lightbars each including a plurality of LEDs coupled in series.
2. Description of the Related Art
FIG. 1 is a diagram illustrating a conventional LED current balance circuit for a single lightbar. Referring to FIG. 1, a lightbar 11 includes a plurality of LEDs D1-Dn coupled in series, where n is a positive integer. A forward voltage Vf1 of the lightbar 11 is a total sum of forward voltages of the LEDs D1-Dn. The lightbar 11 has a first terminal coupled to a lightbar voltage VBUS and a second terminal coupled to an LED current balance circuit 1. The LED current balance circuit 1 includes a transistor Q, a resistor R and an operational amplifier OP. The resistor R detects a current through the lightbar 11 and accordingly generates a detecting result signal. The operational amplifier OP has an inverting terminal for receiving the detecting result signal (corresponding to the current through the lightbar 11), a non-inverting terminal for receiving a setting signal Vset (corresponding to a desired current) and an output terminal for outputting a control signal according to the difference between the current through the lightbar 11 and the desired current.
The transistor Q adjusts, according to the control signal outputted from the operational amplifier OP, its operating point to adjust a voltage drop across the transistor Q to further adjust the forward voltage Vf1 to cause the current through the lightbar 11 to follow the desired current. Therefore, a brightness of the lightbar 11 (corresponding to the current through the lightbar 11) is kept substantially constant and equal to a desired brightness (corresponding to the desired current) regardless of a tolerance of the forward voltage Vf1 of the lightbar 11 which is a total sum of tolerances of the forward voltages of the LEDs Dl-Dn even though they are manufactured in the same batch and the same process by the same manufacturer.
FIG. 2 is a diagram illustrating a conventional LED current balance circuit for a plurality of lightbars. Referring to FIG. 2, a plurality of lightbars 11-1m are employed, and each lightbar 1i is constructed such as the lightbar 11 shown in FIG. 1, where m is a positive integer and i is an integer from 1 to m. Each lightbar 1i has a first terminal coupled to a lightbar voltage VBUS and a second terminal coupled to a current balance unit (not shown) constructed such as the LED current balance circuit 1 shown in FIG. 1. The LED current balance circuit 2 includes a direct-current to direct-current (DC/DC) converter 21 and an LED controller 22. The DC/DC converter 21 converts a DC input voltage VIN to the lightbar voltage VBUS. The LED controller 22 includes the m current balance units coupled to the lightbars 11-1m through a plurality of channel terminals CH1-CHm. Accordingly, a current through each lightbar 1i will follow a desired current such as the desired current (corresponding to the setting signal Vset) shown in FIG. 1. In other words, the currents through the lightbars 11-1m are kept substantially constant and equal to each other to achieve current balance. However, as the number of the lightbars 11-1m increases, the number of the m current balance units increases, resulting in a cost and circuit size increase, and the effect of current balance becomes worse due to the increase in the number of the current balance units having their respective tolerance.
Recently, many specific-purpose integrated circuits (ICs) for the LED controller 22 have been developed. The specific-purpose LED controller IC integrates a fixed number of the current balance units and other functional units. For instance, a functional unit (not shown) in the LED controller 22 outputs a feedback signal through a feedback terminal FB to control the DC/DC converter 21 to adjust the lightbar voltage VBUS to provide feedback control to optimize the lightbar voltage VBUS applied to the lightbars 11-1m. Although the specific-purpose LED controller IC has the advantage of more accurate control and smaller circuit size, it has the disadvantage of higher cost, lower reliability and limited current and power ratings (typically less than 60mA). In a high-voltage and large-current LED lightbar application, there is a need for the specific-purpose LED controller IC to employ an external control manner by using some external components such as m transistor and m resistor for m lightbars so that cost and circuit size also increase as the number of the lightbars increases.